The present invention concerns new aromatic polyimide compositions having a particularly high solubility in many organic solvents and excellent adhesive properties. These compositions may be used as binding agents in the manufacture of composite materials, as insulating varnishes for the coating and enameling of metal conductors, as base for elaborating adhesive compositions, foams and cellular materials and as protecting films able to withstand high temperatures.
The invention concerns more particularly the polymer compositions obtained by reacting an aromatic diamine with a diacid-diester or a tetra-ester derived from benzhydrol 3,3',4,4'-tetracarboxylic acid at a sufficiently high temperature to produce the formation of aromatic polyimides.
It is known that aromatic polyimides are polymers generally infusible and not or not very soluble in the organic solvents. For this reason, it is necessary to make use of operating techniques which either require an intermediate non cyclized soluble polymer or a mixture of fusible reactants which, by a suitable thermal treatment, may give a polymer material.
In the first technique, the polyimide is prepared in two successive steps. During the first step, a reaction of polycondensation between an aromatic diamine and a tetracarboxylic acid dianhydride in a polar aprotic organic solvent gives a solution of polyamide-acid of high molecular weight. This solution must be kept in the cold and protected from moistness since it is sensitive to heat and to hydrolysis.
The second step of the manufacturing process consists of making use of a solution generally containing from 10 to 20% by weight of polyamide-acid in order to prepare a polymer film by progressive evaporation of the solvent. The transformation of the polyamide-acid to a polyimide film is effected by thermal or chemical dehydration. It is obvious that this technique limits the use of polyimides to the manufacture of films or coating materials of small thickness since, in addition to the removal of 80 to 90% by weight of the solvent, it is also necessary to eliminate one mole of water formed by cyclization, for each reactive center.
This technique is illustrated by the U.S. Pat. No. 3 939 109, according to which the dianhydride of benzhydrol tetracarboxylic acid is reacted with 4,4'-diaminodiphenylether to prepare a solution of polyamide-acid in dimethylacetamide. This solution is then used to manufacture polyimide films of a 0.025 mm thickness. Now, as indicated in the U.S. Pat. No. 3 293 267, benzhydrol dianhydride is liable to react with its own alcohol groups to give esters which, on the one hand, change the stoichiometry of the reactants and, on the other hand, decrease the thermal stability of the polymers.
The general method for the preparation of aromatic polyimides, as above described, is, in the present state of the art, the most broadly used in spite of the restraints associated thereto. As a matter of fact, it requires monomers of very high purity, the severe elimination of water, a limited solvent selection, very critical conditions for the synthesis and the preservation of the polymers. With thermal treatments adapted to the solvents and to the polymers, it is however possible to make use of polyamide-acids for synthesizing aromatic polyimides usable as films, varnishes and adhesives.
Another known method for the preparation of aromatic polyimides consists of replacing the tetracarboxylic acid dianhydrides by their reaction products with primary aliphatic alcohols, i.e. by the corresponding alkyl diesters or tetraesters. This technique is described by V. L. Bell (Polymer Letters, 1967, 5, 941-946) and in the U.S. Pat. No. 3 700 649, for the alkyl diesters or pyromellitic and benzophenone 3,3',4,4'-tetracarboxylic acids and various aromatic diamines. The monomer mixture is converted to polyimides by progressive heating under nitrogen atmosphere up to 300.degree. C. As the polycondensation reaction is effected at the melting of the reactants, only the monomers which give fusible polymers may lead to high polymers. In the other cases insoluble and infusible oligomers of low molecular weight are formed, as shown in the comparative examples 1 and 2 of the present invention.
This technique is also illustrated by the French Pat. No. 2 031 847 according to which the esters of meta-phenylene bis-4-(hydroxymethylene phthalic) acid are reacted with aromatic diamines to prepare soluble polyimides. In this case the solubility of the polymers is obtained by an increase of the number of carbinol linkages in the recurrent unit of the polymer, which considerably limits the field of application of these products, since the preparation of the basic monomer requires five successive operations whose total yield is about 50%. In addition, the increase of the number of carbinol linkages results in a decrease of the thermal stability and of the resistance to oxidation of the polyimides. This phenomenon is well known in all the families of heterocyclic polymers where the addition of flexible linkages is used to increase the solubility.